Anti-death penalty protests took place outside the Huntsville Unit in Texas earlier this week. Photograph: David J. Phillip/AP

Reverend Carroll Pickett served from 1980 to 1995 as the chaplain of Huntsville prison in Texas where the state's death chamber is located. He was present when 95 prisoners were executed. Here he describes what he witnessed and why it inspired him to campaign against the death penalty

On execution days, I would join the condemned prisoner from 6am and stay with him until he died at midnight or sometimes later. I walked with him the eight steps it took to get from his cell to the death chamber.

There were two phones in the room and we would wait to see if either one of them rang with a court order to stay the execution. They didn't ring very often - not in the state of Texas.

The first execution I attended was that of Charlie Brooks on 7 December 1982. He was the first person to be killed by Texas after the death penalty resumed in the US in 1976, and the first anywhere in the world to die by lethal injection.

I couldn't find anybody to advise me what to expect, as it was new to everybody. That was hard, the not knowing.

After I had attended a few executions I developed a procedure. I would spend time with the condemned man working out what their last words would be so that I could pass the information on to the warden and make sure the killing wasn't started until the prisoner had finished speaking.

When they were inside the death chamber they all wanted to maintain contact, they wanted me to hold their hand. But you can't do that because they are strapped and taped down.

Instead I would stand right next to them and put my hand on their right leg where I could feel a pulse. That way, they always knew someone was with them to the very end.

I felt that God had put me there because everybody needs a friend. I used to call what I did the ministry of presence.

I would stay with them until the funeral home arrived to pick up the body. That could be late into the night. I remember spending 26 hours straight with one man who was taken to the death chamber, then given a stay of execution, then taken back to the death chamber, over and over again until just before sunrise.

In the end he got a reprieve, and was sent back to death row. But a year later they brought him back and executed him at four in the morning. That was a form of cruel and unusual punishment, and though it's banned by the US constitution I've seen many people go through it.

Texas has made several changes to the death protocol since I was at my last execution in 1995. Today they carry out the killing at 6pm, not at midnight, and they use one drug rather than three, though that seems to me to be a step backwards as it takes longer to declare the person dead.

I never knew what was going on inside a prisoner's head as the drugs were injected into their veins. I remember one man saying "It's burning" and another "It hurts".

Many of the men confessed to me their crimes shortly before they died. At around ten minutes to midnight, when they realised there was no more hope and death was inevitable, they would want to tell me everything. It was traumatic because they might tell me about crimes that no one knew about but I could do nothing with the information.

I know for a fact that I watched four innocent men being killed by the state of Texas, and many more men die who should never have been sent to the chamber. I've calculated that of the 95 some 35 were "fall partners" - that is, there were two or more people involved in the crime and they were not the one who pulled the trigger.

Of the innocent men, Carlos DeLuna was the hardest for me because I knew he had done nothing wrong. What was striking about him and the other innocent men was that they were the most peaceful at the point of their deaths.

They knew they had tried every single legal route to save themselves, and that there was nothing more to be done. So they went to their deaths calmly and without anguish. Carlos de Luna expressed nothing but love in his final moments.

He didn't have a father and on the last day of his life he started calling me Daddy. As he was strapped to the table, he looked right up at me and said it: "Daddy". That hurt a lot.

I've come to see the death penalty as totally wrong, for so many different reasons. Here's one: how can Texas kill people to teach other people that killing people is wrong?

Another reason is the knowledge that innocent men were put to death. Years after I stood beside Carlos de Luna as he was given the lethal injection, a professor at Columbia University proved beyond any doubt that he had been confused with another man called Carlos and his execution was all in error.